The invention relates to a low-pressure mercury vapor discharge lamp provided with a discharge vessel,
which discharge vessel encloses a discharge space containing a filling of mercury and a rare gas in a gastight manner,
which discharge vessel contains an amalgam which is in communication with the discharge space,
and in which the low-pressure mercury vapor discharge lamp comprises discharge means for maintaining an electric discharge in the discharge vessel.
In mercury vapor discharge lamps, mercury constitutes the primary component for (efficiently) generating ultraviolet (UV) light. A luminescent layer comprising a luminescent material (for example, a fluorescence powder) may be present on an inner wall of the discharge vessel for converting UV to other wavelengths, for example to UV-B and UV-A for tanning purposes (sun panel lamps) or to visible radiation for general purposes of illumination. Such discharge lamps are therefore also referred to as fluorescence lamps. The discharge vessel of low-pressure mercury vapor discharge lamps is usually circular and comprises both elongated and compact embodiments. Generally, the tubular discharge vessel of compact fluorescence lamps has a collection of comparatively short straight parts of a comparatively small diameter, which straight parts are interconnected by means of bridge parts or via bent parts. Compact fluorescence lamps are usually provided with an (integrated) lamp base. In such embodiments of the low-pressure mercury vapor discharge lamp, the discharge means comprise electrodes which are arranged in the discharge space. An alternative embodiment comprises the electrodeless low-pressure mercury vapor discharge lamps.
The term xe2x80x9cnominal operationxe2x80x9d in the description and claims of the present invention is used for indicating operating conditions in which the mercury vapor pressure is such that the radiation output of the lamp is at least 80% of the output during optimum operation, i.e. under operating conditions where the mercury vapor pressure is optimal. The amalgam limits the mercury vapor pressure in the discharge vessel as compared with the discharge lamp containing only free mercury. This renders nominal operation of the lamp possible at comparatively high lamp temperatures such as may occur in the case of a high lamp load, or when the lamp is used in a closed or badly ventilated luminaire. Furthermore, the term xe2x80x9cinitial radiation outputxe2x80x9d in the description and claims is defined as the radiation output of the discharge lamp 1 second after switching on the discharge lamp and the xe2x80x9crun-up timexe2x80x9d as the time which the discharge lamp requires for achieving a radiation output of 80% of that during optimum operation.
A low-pressure mercury vapor discharge lamp as described in the opening paragraph, hereinafter also referred to as vapor pressure-controlled lamp, is known from U.S. Pat. No. 4,093,889. The mercury vapor pressure at room temperature is comparatively low in the known lamp. The known lamp thus has the drawback that, when it is operated on a conventional lamp supply, the initial radiation output is also comparatively low. Moreover, the run-up time is comparatively long because the mercury vapor pressure rises only slowly after switching on the lamp.
In addition to the amalgam lamps described above, low-pressure mercury vapor discharge lamps are known which do not only comprise a (main) amalgam but also an auxiliary amalgam. Provided that the auxiliary amalgam contains sufficient mercury, the lamp will have a comparatively short run-up time. Upon switching on the lamp, the auxiliary amalgam is heated by the electrode so that it evolves a substantial portion of the mercury present therein comparatively quickly. It is desirable that the lamp must have been out of operation for a sufficiently long time before switching on, so that the auxiliary amalgam has been able to take up sufficient mercury. If the lamp has been out of operation for a comparatively short period, the shortening effect on the run-up time is only weak. In addition, the initial radiation output is (even) lower than that of a lamp with a main amalgam only because the auxiliary amalgam sets a comparatively lower mercury vapor pressure in the discharge space. Furthermore, the drawback arises in comparatively long lamps that comparatively much time is required before the mercury evolved by the auxiliary amalgam has spread over the entire discharge vessel, so that such lamps show a comparatively bright zone near the auxiliary amalgam and a comparatively dark zone remote from the auxiliary amalgam for a few minutes after switching on.
Furthermore, low-pressure mercury vapor discharge lamps are known which are not provided with amalgam and contain exclusively free mercury. These lamps, hereinafter also referred to as mercury lamps, have the advantage that the mercury vapor pressure at room temperature and hence the initial radiation output are comparatively high. Moreover, the run-up time is comparatively short. Also comparatively long lamps of this type have an approximately constant brightness substantially throughout the length after switching on, because the vapor pressure (at room temperature) is sufficiently high upon switching on. Nominal operation at comparatively high lamp temperatures can be achieved with a mercury lamp whose discharge space contains (just) enough mercury to establish a mercury vapor pressure at the operating temperature, which mercury vapor pressure is close to the optimum mercury vapor pressure. During the lifetime of the lamp, however, mercury is lost because this is bound, for example, on a wall of the discharge vessel and/or on emitter material. In practice, such a lamp thus has only a limited lifetime. In mercury lamps, a quantity of mercury is therefore dosed which is considerably higher than the quantity required in the vapor phase during nominal operation. However, this has the drawback that the mercury vapor pressure is equal to the vapor saturation pressure associated with the temperature of the coldest spot in the discharge vessel. Since the vapor saturation pressure rises exponentially with the temperature, temperature variations which occur, for example, in a badly ventilated luminaire or in the case of a high lamp load, lead to a decrease of the radiation output. At comparatively low ambient temperatures, the mercury vapor pressure decreases, which also leads to a decrease of the radiation output.
It is an object of the invention to provide a lamp low pressure mercury vapor discharge which, at least in regular use, has a comparatively high initial radiation output and a comparatively short run-up time, and also a comparatively high radiation output in a comparatively large range of ambient temperatures.
According to the invention, the amalgam has a bismuth-tin ratio (Bi:Sn) in the range of 20:80xe2x89xa6Bi:Snxe2x89xa680:20, a lead content (Pb) in the range of 0.7xe2x89xa6Pbxe2x89xa612 at % and a mercury content (Hg) in the range of 0.05xe2x89xa6Hgxe2x89xa62 at %.
An advantage of the use of such an amalgam is that, at room temperature, the mercury vapor pressure is comparatively close to that of liquid mercury. With said composition of the amalgam, the discharge lamp is nominally operated at a corresponding temperature of the coldest spot in the discharge vessel, ranging between comparatively wide temperatures of 65xc2x0 C. to 140xc2x0 C. A further advantage of the use of such an amalgam is that the curves at which the mercury vapor pressure is plotted as a function of the temperature can be adjusted via the mercury content and/or the composition of the amalgam. Said properties of the (main) amalgam, namely the broad temperature interval and the variable mercury vapor pressure curves are realized by the choice of the composition of the amalgam according to the invention. For the amalgams with a composition according to the invention, curves in which the mercury vapor pressure is plotted as a function of the temperature have a first stabilization range in the temperature range under the ternary Bixe2x80x94Snxe2x80x94Pb eutectic (at 100xc2x0 C.). In said stabilization range, these curves are at least substantially independent of the mercury content and the composition of the amalgam. The latter property is mainly caused by the fact that the number of phases in the corresponding temperature interval is equal to the number of components, which results in the mercury vapor pressure being mainly only a function of the temperature. In the description of the present invention, the term xe2x80x9cstabilization rangexe2x80x9d is understood to mean a temperature range within which the mercury pressure (PHg) is at least substantially constant.
The mercury vapor pressure versus temperature curves of the amalgam with a composition according to the invention comprise a second stabilization range which is present in the temperature range above the ternary Bixe2x80x94Snxe2x80x94Pb eutectic and below the binary Bixe2x80x94Sn eutectic. Since the number of phases is smaller than the number of components in said second stabilization range (above the eutectic point), the mercury vapor pressure is a function of both the temperature and of the composition of the amalgam, particularly of the mercury content and the lead content of the amalgam. The result is that low-pressure mercury vapor discharge lamps provided with an amalgam having a composition in accordance with the invention combine a satisfactory initial radiation output and a comparatively short run-up time with a comparatively broad interval, at nominal operation, for the temperature of the coldest spot in the discharge vessel. Nominal lamp operation is thus possible in a comparatively large temperature interval.
A further advantage of the use of the amalgam according to the invention is that the amalgam is usable in low-pressure mercury vapor discharge lamps that can be dimmed.
An embodiment of the low-pressure mercury vapor discharge lamp in accordance with a first aspect of the invention is characterized in that, in the amalgam, the bismuth-tin ratio is 20:80xe2x89xa6Bi:Snxe2x89xa680:20, the lead content is 0.7xe2x89xa6Pbxe2x89xa612 at % and the mercury content is 0.2xe2x89xa6Hgxe2x89xa62 at %.
With this composition of the amalgam, at least 80% of the radiation output (nominal operation) of the low-pressure mercury vapor discharge lamp is achieved, in operation, at a corresponding temperature of the coldest spot in the discharge vessel, within a comparatively wide temperature range of 65xc2x0 C. to 140xc2x0 C., while at least 90% of the radiation output is achieved at a corresponding temperature of the coldest spot, within a comparatively wide temperature range of 70xc2x0 C. to 130xc2x0 C. The run-up time of the discharge lamp with an amalgam in accordance with a first aspect of the invention is less than ten minutes in both cases, and in the presence of an auxiliary amalgam, the run-up time decreases to less than three minutes. Amalgams having a composition in accordance with a first aspect of the invention are notably suitable for use in (energy-saving) (compact) low-pressure mercury vapor discharge lamps. Such discharge lamps have a satisfactory initial radiation output and combine a comparatively short run-up time with a comparatively broad interval for the temperature of the coldest spot in the discharge vessel during nominal operation. Consequently, nominal lamp operation is possible within a comparatively large temperature interval.
In a preferred embodiment of the low-pressure mercury vapor discharge lamp in accordance with a first aspect of the invention, the bismuth-tin ratio in the amalgam is 30:70xe2x89xa6Bi:Snxe2x89xa670:30, the lead content is 1xe2x89xa6Pbxe2x89xa610 at % and the mercury content is 0.25xe2x89xa6Hgxe2x89xa61.2 at %.
With this composition of the amalgam, at least 80% of the radiation output (nominal operation) of the low-pressure mercury vapor discharge lamp is achieved, in operation, at a corresponding temperature of the coldest spot of the discharge vessel within a comparatively wide temperature range of 70xc2x0 C. to 170xc2x0 C., while at least 90% of the radiation output is achieved at a corresponding temperature of the coldest spot within a comparatively wide temperature range of 75xc2x0 C. to 160xc2x0 C. The run-up time of the discharge lamp is less than ten minutes in both cases, and in the presence of an auxiliary amalgam, the run-up time decreases to less than three minutes. The result is that (energy-saving) (compact) low-pressure mercury vapor discharge lamps provided with an amalgam in accordance with a first aspect of the invention combine a satisfactory initial radiation output and a comparatively short run-up time with a comparatively very wide interval, during nominal operation, for the temperature of the coldest spot in the discharge vessel. Nominal lamp operation is thus possible within a comparatively very large temperature interval. For a suitably chosen Hg content (for example, 0.5 at % Hg), the temperature of the coldest spot during nominal operation of the discharge lamp may be as high as 180xc2x0 C.
In accordance with a second aspect of the invention, a low-pressure mercury vapor discharge lamp of the type described in the opening paragraph is therefore characterized in that the bismuth-tin ratio in the amalgam is 20:80xe2x89xa6Bi:Snxe2x89xa680:20, the lead content is 0.7xe2x89xa6Pbxe2x89xa612 at % and the mercury content is 0.05xe2x89xa6Hgxe2x89xa60.5 at %.
With this composition of the amalgam, at least 80% of the radiation output (nominal operation) of the low-pressure mercury vapor discharge lamp is achieved, in operation, at a corresponding temperature of the coldest spot in the discharge vessel within a comparatively wide temperature range of 65xc2x0 C. to 140xc2x0 C. Amalgams in accordance with a second aspect of the invention are notably suitable for use in electrodeless lamps.
In a preferred embodiment of the low-pressure mercury vapor discharge lamp in accordance with a second aspect of the invention, the bismuth-tin ratio in the amalgam is 30:70xe2x89xa6Bi:Snxe2x89xa670:30, the lead content is 1xe2x89xa6Pbxe2x89xa610 at % and the mercury content is 0.05xe2x89xa6Hgxe2x89xa60.5 at %.
With said composition of the amalgam, at least 80% of the radiation output (nominal operation) of the low-pressure mercury vapor discharge lamp is achieved, in operation, at a corresponding temperature of the coldest spot of the discharge vessel within a comparatively wide temperature range of 70xc2x0 C. to 170xc2x0 C.
In addition to said materials, the amalgam according to the invention may also comprise additions of, for example, zinc, silver, gallium, indium and/or other elements. It is desirable that such additions shift the melting range (100xc2x0 C. to 140xc2x0 C.) of the Bixe2x80x94Snxe2x80x94Pb alloys by not more than 20xc2x0 C.
At the start of the lifetime of a low-pressure mercury vapor discharge lamp, a comparatively large quantity of mercury may be bound on the wall during operation. To avoid this, the discharge vessel of a lamp according to the invention may have a protective coating of a metal oxide on an internal surface. Such a protective coating, for example, of scandium oxide, yttrium oxide, lanthanum oxide or of an oxide of one of the lanthanides inhibits loss of mercury through binding on the wall. It is favorable when the discharge lamp consumes a small amount of mercury so that the amalgam can be designed in a more optimum way.
These and other aspects of the invention are apparent and will be elucidated with reference to the embodiments described hereinafter.